I can sympathize with this narrative and it is pretty much verified at this point that the "reputation management" industry is a very real scourge on the Internet today. I suspected this was involved back when I saw the flood of comments and opinion pieces villifying and blaming Sanders and Stein for Trump's victory.
On this matter though I have to respectfully disagree with your assessment here, mostly based on a good deal of thought on real life experiences that I have. My wife has always disliked Assange and she jumped on this bandwagon quite willingly of demonizing him. I can assure you I know a thing or two about her, and I can promise she is not part of any PR campaign she is aware of. Let me offer a simpler explanation.
If anything this election season has taught myself and many people is that there are a lot of powerful entities with a vested interest in public opinion being swayed one way or another. Each side is accusing the other of fake news and both sides are using various disingenuous tactics and journalistic practices to confirm this on all sides. People simply are skeptical of everything. Human nature is such that not having conviction in your beliefs is stressful. We may be on the fence, unsure of what to believe, and we will be hyper focused looking for a tiny little piece of evidence to send us over the edge to a strong conviction.
Julian Assange has been a net positive benefit for society in general. There is also a huge smear campaign against him that is hard to verify. We all suspect that the rape charges against him may have been a snare to get him under control and punish him, but we can't know for sure that he didnt in fact commit these crimes. This statement in light of his tweet shows us something concrete and negative about his character that millions of people will have an overblown and visceral reaction to because it is so clear and easily verified. There is no doubt what was said in his tweet versus what is said by his lawyer. This was that one small piece of evidence that people were looking for to give them the conviction of belief they so desperately want.